IT202100002465A1 - PROCESS FOR THE EXTRACTION OF RESIDUAL CHEMICALS FROM A POLYMER MATRIX. - Google Patents

Description

PROCESS FOR THE EXTRACTION OF RESIDUAL CHEMICALS FROM A POLYMER MATRIX.

The present invention relates to a process for the extraction of residual chemical substances from a polymeric matrix. Pi? in particular, the present invention relates to a process for the extraction of residual chemical substances, in particular metals, from a crosslinked elastomeric matrix.

The disposal and recovery of polymeric materials ? a deeply felt need in order to reduce the impact of these materials on the environment due to their non-biodegradability. Among the most polymeric materials? widespread and which pose the greatest problems from the point of view of their recycling are the cross-linked elastomeric materials, which cannot be melted or easily degraded. Technologies have been developed for the recycling of these materials, usually mixed with virgin polymers, after being reduced to fine powder through complex mechanical grinding or chemical devulcanization processes.

Alternatively, the cross-linked elastomeric materials, reduced to a suitable divided form, for example by granulation, can be used as fillers for the creation of walkable surfaces or for the production of manufactured articles to be used in various fields, such as construction and Street furniture.

However, these materials can present further problems in terms of environmental safety, as they contain, in addition to the polymeric material, residual chemicals which largely derive from the crosslinking process which requires the addition of numerous products, such as crosslinkers, accelerators, activators , inhibitors, stabilizers, antioxidants, process aids, adhesion promoters, etc. There? ? particularly true for sulfur cross-linked elastomeric materials, widely used for the production of articles where ? very high resistance to wear is required, such as tyres, flooring, drive belts, etc.

A class of residual chemicals present in elastomeric materials, which are difficult to extract when such material is reticulate, ? made up of polycyclic aromatic hydrocarbons (PAH), which tend to bind in a substantially stable way to the components present in the polymeric matrix, so they are difficult to extract through the use of organic solvents, even when these are combined with ultrasound and/or heat.

Patent CN 106349498 describes a process for the treatment of a recycled rubber particulate which involves the use of carbon dioxide in supercritical conditions, which? able to penetrate inside the elastomeric matrix and extract the PAHs and other organic molecules present. The extraction is carried out at a temperature of 50?C to 150?C and at a pressure of 8 MPa to 50 MPa, and can be further aided by the addition, in combination with the fluid under supercritical conditions, of a solvent, such as water, acetone, toluene, trichloromethane, dichloromethane, petroleum ether, nexane, n-heptane, ethanol or propanol.

Another class of residual chemicals found in elastomeric materials? made up of metals, especially heavy metals such as cobalt and zinc and others. The zinc ? generally present in the vulcanization system, in particular in the sulfur one, which provides for the addition of zinc oxide as a crosslinking activator. On the other hand, cobalt-based organometallic compounds are widely used as adhesion promoters of rubber to reinforcing materials, in particular to brass-plated steel cords.

The removal of metals is difficult as to date there are no known non-destructive methods for the extraction of metals from a polymeric matrix. The extractability of metals? in fact strongly hindered by the steric bulk of the organometallic compounds and by the nature of the chemical bonds which are established with the polymeric matrix.

Is the Applicant yes? posed the problem of finding a process for the extraction of metals from a polymeric matrix, in particular from a crosslinked elastomeric matrix, which can be implemented without the use of polluting substances and which leads to the obtainment of a granulate of elastomeric material having a reduced content of residual metals and can therefore be used for various uses, for example as a filler for sports facilities, for children's playgrounds, and other applications that require frequent and/or continuous contact with the skin of the users.

The Applicant has found that this and other problems better illustrated hereinafter can be solved by means of an extraction process which uses a supercritical fluid, in particular supercritical carbon dioxide, and at least one complexing agent. In fact, it is believed that the supercritical fluid, thanks to its very low surface tension, is able to permeate inside the polymeric matrix and to facilitate the entry of the complexing agent, which penetrates the material and forms complexes with the metals present which they come like this removed relatively quickly, without leaving residues in the material that could be dangerous.

The Applicant has found that the above extraction process allows to remove, in addition to the metals, also the more organic compounds? furniture, commonly present in polymeric materials, cos? to reduce or eliminate the typical smell that characterizes these materials, especially when it comes to cross-linked materials.

The Applicant has also found that this extraction process can be combined with a phase of extraction of residual organic substances through the same supercritical fluid in the presence of a cosolvent, so? to significantly reduce the concentration of residual chemicals of a less mobile organic nature, such as PAHs.

The present invention therefore relates to a process for the extraction of residual chemicals from a metal-containing polymeric material, which comprises:

arranging the polymeric material in partitioned form in an extraction container;

introducing a supercritical fluid into the extraction container, where pressure and temperature conditions are maintained above the critical pressure (Pc) and critical temperature (Tc) of the fluid used; maintain contact of the polymeric material in divided form with the supercritical fluid in the presence of at least one complexing agent, so to extract the metals from the polymeric material;

extract the supercritical fluid containing the metals from the container, and reduce pressure and temperature so? to bring the supercritical fluid to the gaseous state.

Preferably, the polymeric material ? a crosslinked elastomeric material, in particular a sulfur crosslinked elastomeric material. This can derive from the recycling of end-of-life products, such as tyres, transmission belts, pipes, flooring, gaskets, cables. It is separated from any other materials present (in particular reinforcing elements, such as metal or synthetic fiber cords), and then reduced to a subdivided form, in particular in granules or powder, having an average size preferably not exceeding 10 mm , more preferably not exceeding 4 mm.

The metals present in the polymeric material are generally in ionic form, or in the form of oxides or sulphides. These metals are mainly heavy metals such as cobalt, zinc, lead. Other metals can be aluminum, magnesium or mixtures thereof. These are metals often present in some elastomeric compounds.

Preferably, the supercritical fluid ? supercritical carbon dioxide. The critical pressure and temperature values of carbon dioxide are: Pc = 73.8 bar (7.38 MPa); Tc = 31.1?C. Therefore, pressure and temperature values higher than Pc and Tc are maintained in the extraction container, preferably a pressure from 80 bar to 500 bar, more preferably from 100 bar to 400 bar, and a temperature from 50?C to 200?C, more? preferably from 80?C to 150?C.

As for the complexing agent, this is? generally an organic compound containing at least one electron donating atom or group, e.g. nitrogen, carbonyl group, carboxyl group, phosphate group, sulfinyl group. Preferably, the complexing agent contains at least two electron donating atoms or groups.

Preferably, the complexing agent ? selected from: ?-diketones, diethylenetetraminoacetic acid (EDTA) or its salts, dithiocarbamates, trialkyl phosphates (in particular tributyl phosphate-nitric acid complex). Particularly preferred are the ?-diketones, pi? especially acetylacetone.

The quantity? of complexing agent to be used in the process according to the present invention can? vary within very wide limits, mainly as a function of the nature of the agent itself, of the quantity? of metals to be extracted and the contact time with the material to be treated. For example, in the case of acetylacetone used to treat a typical sulfur crosslinked material for tyres, on the basis of the experimentation carried out by the Applicant on a laboratory scale, ? usable quantity? of acetylacetone preferably from 0.2 g to 5 g, plus? preferably from 0.4 g to 2.5 g, per g of treated material.

In another preferred embodiment, the complexing agent is introduced into the extraction vessel together with the supercritical fluid.

The extraction container ? a sealed container that ? able to withstand high pressures, for example an autoclave.

As for the supercritical fluid, this is preferably introduced into the extraction container with a speed? flow that can? vary within wide limits according to the characteristics of the extraction plant and the material to be treated. Generally, based on the experimentation carried out by the Applicant on a laboratory scale, the speed? of flow ? between 5 g/min and 100 g/min, preferably between 10 g/min and 50 g/min.

The polymeric material to be treated can? be kept in contact with the supercritical fluid for variable times depending, for example, on the chemical-physical characteristics of the material to be treated, on the pressure and temperature at which the extraction is carried out, on the quantity of metals that you intend to extract, of the speed? of flow of the supercritical fluid, of the effectiveness of the complexing agent. In general, based on the experimentation carried out by the Applicant on a laboratory scale, the contact time can vary from 10 min to 240 min, preferably from 30 min to 180 min.

The extraction process according to the present invention can be made continuously or in batches. Upon exiting the extraction container, the pressure and temperature of the fluid are reduced so as to reach subcritical conditions, thus to obtain the gasification of the fluid, while the complexed metals are collected and disposed of.

In a preferred embodiment, the process according to the present invention further comprises contacting the polymeric material in partitioned form with the supercritical fluid in the presence of at least one co-solvent, thus to extract residual organic substances from the polymeric material, in particular polycyclic aromatic hydrocarbons (PAH), aromatic amines, and others.

This extraction phase can be carried out simultaneously with the metal extraction phase, or before or after this phase. It is believed that, in order to avoid possible interactions between the two types of extraction, it is advantageous for the phase of extraction of the residual organic substances to be carried out separately from the phase of extraction of the metals. Between the two phases pu? It may be advantageous to carry out a cleaning step of the material to be treated by a flow of the supercritical fluid alone, without adding the solvent or the complexing agent.

The PAHs pi? commonly present in crosslinked elastomeric materials, in particular those containing carbon black, are: benzofluoranthenes, pyrene, benzopyrenes, dibenzoantracenes, indenopyrenes, benzoperylene, chrysene, phenanthrenes, fluoranthrene, and the like. As for the aromatic amines pi? commonly found in this type of material, we mention the following: N-(1,3-dimethylbutyl)-N'-phenylp-phenylenediamine (6PPD), N-isopropyl-N'-phenyl-1,4-diphenylenediamine (IPPD), N-(1,4-dimethylpentyl)-N'-phenyl-p-phenyldiamine (7PPD), N,N'-bis(1,4-dimethylpentyl)p-phenylenediamine (77PD), diaryl-pphenylenediamines (DAPD).

As regards the conditions with which to carry out the phase of extraction of the residual organic substances, these can be chosen in the same way as indicated above for the phase of extraction of the metals.

As for the co-solvent, this ? preferably selected from: water, C1-C4 alcohols (preferably methanol or ethanol), C3-C9 ketones, and mixtures thereof. Particularly preferred are the mixtures between water and methanol or ethanol, preferably with a water/alcohol ratio from 90:10 to 60:40 (w/w).

Alternatively or in combination, the co-solvent can be introduced into the extraction container together with the supercritical fluid.

The Applicant has found that, at the end of the extraction, the polymeric material in divided form can present on the surface a whitish patina that pu? be attributed to the presence of substances extracted but not removed from the flow of supercritical fluid. Therefore, can it may be advantageous to submit the polymeric material after the extraction step (or steps) to a washing step, in order to remove the substances deposited on the surface. This washing phase can be carried out simply with water or a solvent, or preferably with an aqueous solution of nitric acid. To improve the effectiveness of washing, this can? be accomplished by sonication.

This invention will come now further described with reference to:

Figure 1, what ? a schematic representation of a plant for carrying out the process object of the present invention.

In Figure 1, the fluid (for example carbon dioxide), coming from a pressurized tank (1), is brought to a pressure higher than the Pc value by means of a pump (2). The fluid is mixed with a complexing agent coming from a tank (3) and also brought to the above pressure by means of a pump (4). The mixture is pre-heated to a temperature higher than the Tc by means of a pre-heater (5), so to bring the fluid to supercritical conditions. This is then placed in an autoclave (6) where ? present the polymeric material to be treated in divided form. Alternatively, the supercritical fluid and the complexing agent can be fed separately to the autoclave (6). As for the possible co-solvent, this pu? be fed to the autoclave (6) separately or, preferably, together with the supercritical fluid.

After the extraction step, the supercritical fluid is taken from the autoclave (6) and its pressure is reduced by passing through a separator (7). The fluid then passes to the gas phase and pu? be recovered and recirculated in the plant, while the liquid part containing the extracted substances is collected in a tank (8).

The following embodiments are provided for illustrative purposes only of the present invention and must not be understood as limiting the scope of protection defined by the appended claims.

EXAMPLE 1 (comparison).

A laboratory-scale plant was set up according to Fig. 1 in the attachment, and in the extraction container, consisting of an autoclave equipped with sealing gaskets such as to guarantee the maintenance of the pressures and temperatures necessary for carrying out the tests, 50 g of a granulate of sulfur-reticulated rubber were introduced (average size of the granules = 3x3x3 mm).

The granulate contained heavy metals (cobalt and zinc) in the quantities? indicated in the first column of Table 1 (granulate as it is). The quantities? were determined by X-ray fluorescence according to the UNI EN 15309:2007 standard.

The granulate as it is? was also subjected to quantitative analysis of the PAHs present, by extraction with a mixture of cyclohexane and toluene in a soxhlet extractor.

The sample of granules ? been subjected to a first extraction test using supercritical carbon dioxide with these extraction conditions:

T = 120?C;

p = 340 bars;

speed? of CO2 flow = 15 g/min.

After letting only the supercritical fluid flow for 10 min, in the container ? A solvent consisting of a water/methanol mixture was introduced (weight ratio = 80:20; flow rate of the mixture: 1.25 ml/min). The extraction? was conducted under the conditions indicated above for 120 min. At the end of the extraction and after depressurization of the system, the granulate? was collected and analyzed by the method indicated above. At the end of the process, the granulate had lost the characteristic smell of sulfur crosslinked rubber.

The concentrations of cobalt and zinc are shown in Table 1. Is it possible? note that the use of the water/methanol mixture in combination with the supercritical fluid does not lead to a significant decrease in the concentration of metals present in the granulate.

The treated granulate ? was also analyzed for PAH content as described above. From the results reported in Table 1, it can be note a substantial reduction in the amount? of PAHs present, which fall below the limits of detection?.

EXAMPLE 2.

Example 1 ? been repeated under the same conditions, but in replacement of the water/methanol mixture ? a flow of acetylacetone (AcAc) was introduced into the pressurized container (flow rate: 1 ml/min). The extraction? was conducted for 60 min. At the end of the process, the granulate had lost the characteristic smell of sulfur crosslinked rubber.

The concentration of cobalt and zinc in the granulate after extraction? shown in Table 1. Yes? observed a reduction in the concentration by weight of cobalt equal to 30.1% and a reduction in the concentration by weight of zinc equal to 24.1%.

EXAMPLE 3.

Example 2 ? been repeated under the same conditions, with the only difference that at the end of the extraction the granulate ? was washed with HNO3 solution (0.1 mol/l) for 1 hour in a sonicator.

The concentration of cobalt and zinc in the granulate after extraction? shown in Table 1. Yes? observed a reduction in the concentration by weight of cobalt equal to 32.5% and a reduction in the concentration by weight of zinc equal to 34.7%.

EXAMPLE 4.

The same granulate treated according to Example 1 with supercritical CO2 and a water/methanol 80:20 w/w mixture? been subjected to a treatment step with supercritical CO2 and AcAc as complexing agent under the same conditions described in Example 3, at which ? followed by washing with a solution of HNO3 as described in the same Example 3.

Table 1 shows the concentrations of cobalt and zinc after this treatment. The quantitative analysis of PAH is not ? been repeated, being the quantity? residue evidently lower than the detection limits? as reported for Example 1.

TABLE 1.

(*) comparative

Claims (16)

1. Process for the extraction of residual chemicals from a metal-containing polymeric material, which includes: arranging the polymeric material in partitioned form in an extraction container; introducing a supercritical fluid into the extraction container, where pressure and temperature conditions are maintained above the critical pressure (Pc) and critical temperature (Tc) of the fluid used; maintain contact of the polymeric material in divided form with the supercritical fluid in the presence of at least one complexing agent, so to extract the metals from the polymeric material; extract the supercritical fluid containing the metals from the container, and reduce pressure and temperature so? to bring the supercritical fluid to the gaseous state. 2. Process according to claim 1, wherein the polymeric material is a crosslinked elastomeric material, in particular a sulfur crosslinked elastomeric material. 3. Process according to any one of the preceding claims, wherein the metals are present in the polymeric material in ionic form or in the form of oxides or sulphides. 4. Process according to any one of the preceding claims, wherein the metals are heavy metals, for example selected from: cobalt, zinc, lead, or mixtures thereof. 5. Process according to any one of claims 1 to 3, wherein the metals are aluminum, magnesium, or mixtures thereof. 6. Process according to any one of the preceding claims, wherein the supercritical fluid is supercritical carbon dioxide. 7. Process according to claim 6, wherein a pressure of between 80 bar and 500 bar, preferably between 100 bar and 400 bar, and a temperature of between 50°C and 200°C, preferably between 80°C are maintained in the extraction container at 150?C. 8. Process according to any one of the preceding claims, wherein the complexing agent is? an organic compound containing at least one electron donating atom or group, e.g. nitrogen, carbonyl group, carboxyl group, phosphate group, sulfinyl group. 9. Process according to claim 8, wherein the complexing agent is? an organic compound containing at least two electron donating atoms or groups. 10. Process according to claim 9, wherein the complexing agent is selected from: ?-diketones, diethylenetetraminoacetic acid (EDTA) or its salts, dithiocarbamates, trialkyl phosphates (in particular tributyl phosphate-nitric acid complex). 11. Process according to claim 10, wherein the complexing agent is acetylacetone. The process according to any one of the preceding claims, wherein the complexing agent is introduced into the extraction container together with the supercritical fluid. The process according to any one of the preceding claims, further comprising contacting the polymeric material in partitioned form with the supercritical fluid in the presence of at least one co-solvent, thus to extract residual organic substances from the polymeric material, in particular polycyclic aromatic hydrocarbons (PAH), aromatic amines, and others. 14. Process according to claim 13, wherein the co-solvent is selected from: water, C1-C4 alcohols (preferably methanol or ethanol), C3-C9 ketones, and mixtures thereof. 15. Process according to any one of the preceding claims, further comprising subjecting the polymeric material after the extraction step (or steps) to a washing step, in order to remove the substances deposited on the surface. 16. Process according to claim 15, wherein the washing step is carried out with an aqueous solution of nitric acid, optionally by sonication.